Lesson 2 Thermodynamics, Kinetics Of Corrosion Flashcards by Shyla Carace

Butler-Volmer equation is used for the determination of the exchange current density of
a material. Which of the following is NOT related or included in the formula?

Viscosity

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In corrosion kinetics, mass transfer is governed by three forces, but two of these are
negligible under some specific conditions. What is the remaining mass transfer force
considered in the corrosion process?

Diffusion

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For the approximation of the diffusion coefficient, tables are available in different handbooks. In case there is none, an equation under Fick’s First Law is utilized. Based on that equation, what is the relationship of the diffusion coefficient with temperature and
viscosity?

Temperature is directly proportional to the diffusion coefficient, while it is the opposite with viscosity

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What does a high activation overpotential indicate?

The metal has increased corrosion
resistance

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The following variables or properties are included in the computation of flux under Fick’s
first law, except:

Faraday Constant

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Calculate the ΔG of this reaction.

Mg + H2O + 1/2 O2 —> Mg(OH)2

-596600

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It is useful relations between the energy and the potential of a cell to the concentrations of
participating ions and other chemical species.

Nernst Equation

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Nernst Equation is named after _____?

Walther Nernst

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A large negative ΔG means a _____?

It can’t be determined

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The more negative the ΔG indicate a______?

Greater tendency of metal to react with its environment.

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This equation describes the relationship between the current density (i) at an electrode and
the overpotential (η) applied to it.

Tafel equation

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Based on the anodic polarization curve, decreasing the Ip will result to?

High resistance to
uniform, galvanic, erosion, stress corrosion cracking

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Lowering the Epit in anodic polarization curve will result lower resistance to pitting.

False

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We plot the Tafel equation with the electric potential on the X-axis and the logarithm of current density on the Y-axis.

False

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Which of the following could result to low passivity?

Low Ep

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In deaerated acid, which of the following metals will suffer corrosion?

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The Pourbaix diagram outline regions of active corrosion, oxides/hydroxides formation, and
thermodynamic immunity for a metal. They are essentially phase diagrams that map the conditions of _______ as a function of ______.

Potential
pH

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It is the half-cell in which hydrogen gas is bubbled over a platinum electrode immersed in a
solution having a known concentration of hydrogen ions.

Standard Hydrogen Electrode

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The Pourbaix diagram can be useful in predicting localized corrosion such as pitting and
stress corrosion cracking.

True

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What will happen when Al3+/Al* and H+/H equilibria are allowed to couple?

The resulting electrochemical cell will give rise to a positive potential only when Al3+/Al equilibrium turns into anodic and H+/H cathodic

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In process of corrosion, a state function called ___ or ___ can possibly be associated

free energy or Gibbs free
energy, G

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In gibbs free energy, G ___ along with the reaction time. It can also be explained that during the reaction
process, there is a driving voltage that is available

decreases

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is the term used to describe this reduction, denoted as ΔG.

Reaction affinity

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is a quantitative measure of a chemical reaction’s tendency to start, which includes the reaction of a metallic material with its environment.

Gibbs free energy change (ΔG)

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a direct measure of the work capacity or the maximum electric energy possible that is available from a system

Gibbs free energy change (ΔG)

When the change in free energy that accompanies a chemical the change of a system from one state to another is **negative**, free energy is said to be ___ and a ____ direction of the system is observed.

lost spontaneous reaction

a **positive** value of Gibbs free energy change means that the transition indicates an energy ___, and it requires additional energy to be added to the system.

increase

For electrochemical reactions, the ___ of the corrosion cells, which are crucial part of the corrosion process, can be used to express the tendency of a metal to corrode.

electromotive force (emf)

1 cal = ___ absolute joules.

4.184

In summary, the change in free energy can be classified as a ___ and is ___ of the reaction path.

state function independent

With regards to electrical and electrochemical processes, ____ can be defined as a product of charges moved (Q) and the electrochemical potential (E) through which it is moved.

electrical work (w)

A higher value of E for any type of cell means there is also a ___ for the overall chemical reaction of the cell to proceed, which is applicable to any type of cell.

higher tendency

A more **negative value of ΔG** results in a ____ to make the reaction proceed

larger tendency

a positive ΔG° value refers to a reaction that has _____.

no tendency to start at all

gold (Au) is an element that does not undergo corrosion in ____ to produce Au(OH)3.

aqueous medium

For a reaction to take place, the _____ must be positive (-ΔG> 0, then ΔG > 0).

driving voltage

When ΔG = 0, it is said that the reaction is said to be in an ______

equilibrium state.

On the other hand, when the initial and final Gibbs free energy are the same (ΔG = 0) or when there is a _____driving voltage (ΔG > 0), there is enough reason to conclude that a reaction might not occur.

negative

Meanwhile, when ΔG > 0, it is said that the system is more likely to ____ to that stated, unless the external energy of the system is affected by external forces.

change in the other direction

It is not ___ that a large negative value of ΔG will result in a higher corrosion rate.

always true

a positive value of ΔG _____ indicates that a reaction will not proceed under certain conditions.

always

The standard conditions for solid, liquid compounds, and even elements is the pure compound or element; in the case of *gases*, it is set at a pressure of ____ and for *solutes*, the ideal concentration is ____.

100 kPa 1 M (mol/L).

the case of galvanic cells like _____, the equation below can be used: Ecell=Ecathode - Eanode

Danielle cells

In _____, the accompanying change in free energy can be determined using knowledge regarding the cell potential of the reaction.

electrochemical or corrosion reactions

The equilibrium electrodes are also termed “______”.

half-cells

If these electrodes are maintained at equilibrium conditions, they are now called "_____”.

standard half-cells

The *positive terminal* is connected to the _____ and the negative terminal is connected to the _____.

Cu electrode (cathode) Zn electrode (anode).

If all the activities of reactants and products are equal to unity, the ____ becomes zero (ln 1 = 0), and ΔG = ΔG°.

logarithm term

used to determine the potential of a system wherein the reactants are not in unit activity.

Nernst equation

The study of reaction rates at the interface between an electrode and a liquid.

Electrode kinetics

an electrode at which a net oxidation process occurs

Anode

an electrode at which a net reduction reaction occurs

Cathode

Synonymous with oxidation reaction (loses electron)

Anodic reaction

synonymous with reduction reaction (gaining electron)

Cathodic reaction

The deviation from equilibrium potential

polarization

measurement of magnitude of polarization with respect to the equilibrium potential of an electrode.

Overvoltage

2 types of polarization

Activation Concentration

____ usually is the controlling factor during corrosion in media containing a **high concentration of active species** (eg. concentrated acids)

Activation polarization

It refers to an electrochemical process that is controlled by the reaction sequence at the metal- electrolyte interface.

Activation polarization

hydrogen reduction on zinc surface.

Activation polarization

generally predominates when the concentration of the **reducible species is small** (eg. dilute acids, aerated salt solutions).

Concentration polarization

It refers to electrochemical reactions that are controlled by the diffusion in the electrolyte.

Concentration polarization

any changes in the system that ___ diffusion rate will ___ the effects of concentration polarization and hence increase reaction rate.

increase decrease

Increasing the velocity or agitation of the corrosive medium will increase rate only if the _____ is controlled by **concentration polarization**.

cathodic process

polarization can lead to the formation of a protective oxide layer on the metal' s surface which acts as a barrier, slowing down further corrosion.

Passivation

Simply the amount of current (I) over a given area (A).

Current density

The total charge that passes through the cell when N moles of the metal M react is Q

Current density, i

current density at equilibrium

EXCHANGE CURRENT DENSITY, i0

when the system isn’t polarized/’overpotential’

EXCHANGE CURRENT DENSITY, i0

a model for the current density of an electrode when the only significant limiting factor is activation

Tafel equation

Tafel equation when simplified

Butler-Volmer Equation

Gibbs free energy determines whether the reaction is ____

spontaneous, non-spontaneous or at equilibrium

quantifies the tendency of any metal's to react with its environment.

CHANGE OF GIBBS FREE ENERGY

Corrosion typically involves two electrochemical reactions:

Oxidation Reduction

metal atoms lose electrons and form metal ions

Oxidation

non-metallic substances such as oxygen gain electrons

Reduction

allows for the flow of ions and electrons between the metal surface and the surrounding environment.

Electrolyte

The overall corrosion reaction can be described in terms of ____ involving oxidation and reduction .

half-reactions

The corrosion of metals occurs within a localized electrochemical cell known as a ____

corrosion cell.

The ___ serves as a medium for ion transport.

electrolyte

The ____ is the potential difference measured under *standard conditions* (1 M concentration, 1 atm pressure, and a temperature of 25°C) for a half-cell reaction

standard electrode potential (E°)

This parameter quantifies the tendency of a metal to undergo oxidation or reduction relative to a standard reference electrode.

standard electrode potential (E°)

It is a historically important electrode. It is the half-cell in which hydrogen gas is bubbled over a *platinum electrode* immersed in a solution having a known concentration of hydrogen ions.

STANDARD HYDROGEN ELECTRODE

Hydrogen equilibrium bears __ value.

zero

Those equilibria having higher standard potential than that of H+/H will be ___ while the others having lower standard potential will be ___ and corrode in acid solutions.

noble Active

In any electrochemical reaction, the *most negative* or active half-cell tends to be ___, and the *most positive* or noble half-cell tends to be ___.

oxidized reduced

Relations between the *energy* and the *potential of a cell* to the concentrations of participating ions and other chemical species

Nernst equation

___, also called ____, outline regions of active corrosion, oxides/hydroxides formation, and thermodynamic immunity for a metal.

Potential-pH diagrams Pourbaix diagrams

In Pourbaix diagrams, if the oxides/hydroxides protect the underlying metal, then this region is called ___

passive region.

__, also known as __ in the context of corrosion kinetics, refers to the deviation in the *potential (voltage) of an electrode from its equilibrium* or *rest potential during an electrochemical reaction*.

Overpotential polarization

It represents the **energy barrier** that must be overcome for the reaction to proceed.

ACTIVATION ENERGY

Activation polarization is due to ___ that are an inherent part of the kinetics of all electrochemical reactions.

retarding factors

The _ is the single most important variable that explains the large differences in the rate of hydrogen production on metallic surfaces.

exchange current density

__ is often added to power cells such as the popular alkaline primary cells to stifle the thermodynamically favored production of gaseous hydrogen and prevent unpleasant incidents.

Mercury

This type of overpotential arises from the variation in **ion concentrations** at the electrode interface.

CONCENTRATION POLARIZATION

is related to the mass transport limitations of reactants or products to or from the electrode surface.

CONCENTRATION POLARIZATION

In conc polarization, the __ term is negligible since it only affects charged ionic species while the _ force disappears in stagnant conditions.

migration convection

The flux of a species O to a surface from the bulk is described with ___

Fick’s first law.